Cold reality is setting in for drivers of Interstate 90, who three years from now might have to pay $1,900 a year to cross Lake Washington.

I-90 tolls would help the state close a deficit of $1.4 billion to complete the new $4.1 billion, six-lane Highway 520 floating bridge, three miles north — a bridge Mercer Island residents say they rarely use.

“I’m really considering selling my house and moving,” said Denise Joffe, a Mercer Island resident. “Why should I be paying a toll for somebody else’s bridge?”

For others, in a vast area from Kirkland to West Seattle, the choice of bridges is more flexible, shown by the fact that 15,000 drivers a day are diverting to toll-free I-90 since the state imposed a toll 13 months ago on the old 520 crossing.

State lawmakers have talked about adding tolls to I-90 since the mid-2000s, but don’t expect to vote until 2014 on a bill to authorize them. After that, installing tolling devices could take another two years. For now, the Washington State Department of Transportation (WSDOT) is taking comments from the public, to decide what issues to explore in an environmental-impact study, costing $2 million.

Joffe was among 800 people at a forum Tuesday night on Mercer Island, where the “No Tolls on I-90” supporters brought an information table, a couple of pickets and a toilet seat to symbolize how their money would be flushed away.

Wednesday’s session in Bellevue drew only 147 people in the first two hours. A third open house is Thursday in Seattle.

A common misconception is that federal law forbids tolling an interstate, but the Federal Highway Administration has said it is open to I-90 bridge tolls, especially if they are designed to control congestion.

Clibborn, who says tolls are inevitable, is at odds with constituents such as Owen Blauman, whose sign said “Keep I-90 a FREE WAY.”

“We don’t have any choice but to pay a toll to get off our island. That’s the fundamental problem,” he said. “It’s a little bit like Alcatraz.”

With an average home value of $830,000, according to Zillow, “the rock” is hardly The Rock. And for many years, it has enjoyed federally funded park lids, along with exclusive access for Mercer Island solo drivers to use the fast-moving I-90 express lanes.

One WSDOT option would split the corridor in half, so toll devices would be mounted over the west and east edges of the island. That way, instead of a $4 toll each way, islanders would pay $2 each way going west into Seattle or east to Bellevue.

Clibborn would prefer one free and one full-rate route off the island.

There’s a Machiavellian cleverness to the tolling strategy: Charge to cross 520, measure the congestion on I-90 when drivers divert, then promote I-90 tolls as an antidote, all to raise money toward 520.

Some toll income should be set aside to improve bus service from Eastgate and Issaquah, said Mark Hallenbeck, director of the Washington State Transportation Center. Clibborn suggested flyover ramps at the I-90/I-405 junction for buses and carpools.

But she warns there won’t be a huge surplus, after covering a still daunting bill to build the Seattle landings of Highway 520.